It is well recognized that chains have been used as securing devices for centuries. The flexibility of the chain and the inability of primitive society to draw long lines or steel cables resulted in the use of chains in most every field. In addition, although most chains were comprised of simple oval or round links, chains having unusually shaped links have also been developed for decoration and other purposes. However, since the primary purpose of the chain was to provide a strong securing device, various methods of treating the steel have been used to provide stronger chains which are less subject to breaking. Such methods are, for example, heat treating of steel having medium to high carbon content, and plastic deformation of low carbon steel at room temperature (cold work). Such treatment almost invariably results in reduced ductility and energy absorption capability. For example, "The Chain Tester's Handbook" by C. H. A. McCaully of Great Britain, and published for the Chain Testers' Association of Great Britain by Rollprint Limited, London, at page 9 includes a table showing the percentage of elongation of various types of steel having different carbon content. As can be seen, dead soft steel having 0.08% carbon has the highest elongation which is 40%.
Other chains include, as an example, chains having twisted links which are commonly used on vehicle tires for providing traction. By use of the twisted link a more regular cross-section is presented between the vehicle tire and the roadway. In the past, although substantially all chains may at times be required to absorb some energy, the goal was generally to increase the strength of the chain. That is, increase its ability to withstand forces (tons, pounds, kilograms, etc.) by making the chain so strong it would not break or deform. Little effort, however, has been made to maximize the energy (foot-pounds, joules, etc.) absorption ability of a chain.
The need for providing some sort of shock or impact (energy) absorption or giving resilience to a chain system has in the past been accomplished by the attachment of resilient members such as hydraulic cylinders, springs, and the like. Other techniques such as the continuous bands disclosed in U.S. Pat. No. 403,147 issued to Schinneller and Jones on May 14, 1889 discloses the use of metal bands having resilience. These bands are used to couple cars such as railroad cars. In addition, U.S. Pat. No. 3,775,969 issued to P. F. Vasterling on Dec. 4, 1973 discloses common chain which has an elastomeric material covering the links and substantially filling the clearance spaces between the links. Thus, assuming that the chain is covered by the elastomeric material while in a contracted condition, the elastomeric material will fill the clearance spaces and thus when the chain is subjected to tension, the elastomeric material between the clearance spaces is compressed such that it must absorb energy as the chain is stretched. Thus, the chain of P. F. Vasterling is an example of a chain system having energy absorbing characteristics. In addition, trailers using chains as safety systems have been combined with springs to provide some energy absorbing capabilities to such combination systems. However, other than such combination sytems, chains having elastomeric material, and resilient bands, chains in the past have been simply built strong enough to assure that tension forces applied thereto cannot exceed the tensile strength of the chain so that the chain retains its original shape without any deformation. Such chains typically are made out of very hard materials such as steel or carbon steel, and are brittle. Therefore, unfortunately, when such high-strength chains fail, they usually do so abruptly. In addition, because of the high strength of the chain itself, the attaching fixtures must also be very strong, since such attaching fixtures should be at least as strong as the chain.
Therefore, it is an object of this invention to provide an energy absorbing chain which allows the use of smaller diameter lower strength links.
It is yet another object of the present invention to provide an energy absorbing chain which is suitable for use in safety systems.
It is still another object of this invention to provide an inexpensive energy absorbing chain which does not require strengthening treatments.
In the past, it has been common to use a chain arrangement with a towed and towing vehicle as a safety device to keep the vehicles together in the event the primary hitch or coupling connecting the two vehicles became disengaged. An example, of such a safety device is disclosed in U.S. Pat. No. 3,123,383 issued to W. J. Humpal on Mar. 3, 1964. The patent to Humpal discloses a standard type safety chain which includes a keyhole link which fits over the ball hitch of a trailer system and in turn attaches to the towed vehicle and the towing vehicle. However, as can be seen, the chain disclosed in this patent uses standard oval shaped links. Similarly, the safety chain system disclosed in the U.S. Pat. No. 3,265,407, issued to C. I. Paddock on Aug. 9, 1966, discloses another safety system which will continue to support the tongue of the towed vehicle such that it doesn't contact the road surface even upon the disengagement of the hitch from the towing vehicle. Again, however, the chain used in this particular safety system uses oval standard shaped links. In addition, the trailer safety chain systems disclosed in the SAE (Society of Automotive Engineers) Handbook, Section 36, covering trailers, trailer coupling hitches and safety chains for automobiles clearly discloses standards for hitches and safety chains recommended by that engineering body. According to paragraph 6.1 of the SAE Standards, a safety chain is defined "as any connection (including chain or its equivalent in stength and the attaching means) from the front of the trailer or trailer tongue to the rear of the towing vehicle for the purposes of retaining connection between the towing and towed vehicle in the event of failure of the trailer coupling or ball". The Standard further goes on to describe specifications of chains which may be used as trailer safety chains. In particular, the specification requires welded steel chain links or their equivalent, wherein the effect of the standard is that each link of a safety chain shall be equal to or greater in minimum break test load than the gross weight of the trailer including its load. Thus, it is seen that to date, safety chain installations have been based upon the concept of a minimum strength test. That is, chains which can withstand great forces, without regard for the energy absorbing capabilities of the chains. Unfortunately, the typical sequence of actions which occur when a towed vehicle becomes unhitched from its towing vehicle can result in greater forces than the weight of the trailer and its content upon the chain. A typical sequence is that when the towed vehicle becomes disconnected from its normal hitch, the driver of the towing vehicle will become aware of the problem perhaps when he sees the towed vehicle moving somewhat uncontrolled at the rear of his towing vehicle. Such uncontrolled movement usually means that the towed vehicle is now being pulled or towed by the safety chains and not the trailer hitch. The driver's usual reaction is to immediately apply the brakes to try to bring the two vehicles under control. However, in many instances, the driver will apply his brakes so hard that the towed vehicle which has no braking system will accelerate rapidly with respect to the speed of the towing vehicle. Thus, it will move forward with its tongue under the vehicle. The increased forces resulting from the towed vehicle's inertia will often snap the safety chain leaving the towed vehicle free to encounter traffic or roadside obstacles. It can be shown with simple calculations using the principles of mechanics that although the safety chain may have had the strength equal to the loaded weight of the towed vehicle, it may be called on to handle even greater forces in an emergency situation when the towing vehicle applies its brakes. To solve this problem, it is more logical that safety chains for such trailer systems be selected such that the safety chain can absorb the kinetic energy due to the inertial forces applied during such a panic situation.
Therefore, it is another object of this invention to provide a trailer safety system having a safety chain which has energy absorption capabilities substantially greater than safety chains of comparable size which are presently available.